In recent years, digital communication technology has been developed for wireless communication apparatuses such as portable phones and smartphones, so that the data transfer is performed with a high efficiency. When a multi-level phase modulation method is used as a method for transmitting data, it is important for a wireless communication apparatus provided on the data transmitting side to inhibit non-linear distortions by linearizing amplification characteristics of a transmission-purpose electric-power amplifier and to reduce the electric power (hereinafter, “power”) that leaks toward adjacently-positioned channels.
Further, in a situation where efforts are exerted to improve the power efficiency by using an amplifier having a low level of linearity, it is desirable to implement a technique for compensating non-linear distortions caused by the low level of linearity. A known example of such a distortion compensation technique is a digital non-linear distortion compensation method called Digital PreDistortion (DPD) method.
In that situation, the wireless communication apparatus may be provided with a distortion compensation unit that compensates the non-linear distortions of the electric power amplifier (hereinafter, “power amplifier”). For example, the distortion compensation unit compares a transmission signal (which may be referred to as a modulation signal or a transmission baseband signal) converted by a Serial/Parallel (S/P) converter into two systems of an I-signal and a Q-signal, with a signal (which may be referred to as a feedback baseband signal) obtained by feeding back a part of the signal resulting from the conversion into the I-signal and the Q-signal and an amplifying process subsequently performed thereon. The distortion compensation unit then calculates a distortion compensation coefficient so that the difference between the two compared signals becomes zero. Further, the distortion compensation unit performs a distortion compensation process by multiplying the transmission signal converted into the I-signal and the Q-signal by the distortion compensation coefficient. The distortion compensation unit compares the transmission signal before the distortion compensation with the signal obtained by feeding back a part of the signal resulting from the amplifying process performed subsequent to the distortion compensation, so as to occasionally update the distortion compensation coefficient in such a manner that the difference between the signals becomes zero.
There are various methods for realizing the DPD process. Generally speaking, Look-up Table (LUT) methods are well known. According to a LUT method, an address is generated based on a parameter (e.g., a signal power) of a transmission signal before the distortion compensation, so that a distortion compensation coefficient in a look-up table is referenced or updated by using the generated address.
Examples of causes of non-linear distortions in output signals (e.g., amplified signals) of a power amplifier include a “memory effect” and an “Idq drift”. The “memory effect” is a voltage fluctuation occurring in the power amplifier caused by a momentary signal change.
The “Idq drift” refers to a phenomenon where a drain bias current (Ids) has a transient response, i.e., a drain current observed immediately after entering into an idling state (i.e., the state where the power amplifier has no RF signal input) becomes lower than an initial set value. The “Idq drift” is a phenomenon unique to a GaN (gallium nitride) device, which is an amplifier that has a high output and a high power efficiency. Further, the larger the output of the power amplifier is immediately before entering the idling state, the more significant is the decrease of the drain current, i.e., the more significant is the “Idq drift”.
For the purpose of compensating the non-linear distortions caused by the “memory effect” and the “Idq drift” explained above, distortion compensation coefficients each of which corresponds to a different one of multiple-dimensional addresses based on mutually-different “parameters” of the transmission signal may be used. For example, as a one-dimensional address, an address based on the power (decibels [dB] or the amplitude) of the transmission signal serving as a “parameter” may be used. As a two-dimensional address, an address based on a power differential value of the transmission signal serving as another “parameter” may be used. As a three-dimensional address, an address based on a power integral value of the transmission signal serving as yet another “parameter” may be used. In these examples, the two-dimensional address contributes to the removal of distortions caused by the memory effect. The three-dimensional address contributes to the removal of distortions caused by the Idq drift.
Incidentally, due to a rapid increase in the traffic in wireless communication systems in recent years, the number of installed wireless base stations is rapidly increasing. Many wireless base station apparatuses these days are configured so as to include one controlling device and a plurality of wireless devices, and in particular, the number of installed wireless devices is rapidly increasing. For this reason, a problem has arisen where there are not enough locations in which the wireless devices are to be installed. One idea for a solution to this problem is to install one wireless device of which the corresponding range of frequencies is expanded, instead of the related method where a plurality of wireless devices respectively corresponding to a plurality of frequencies are installed. For instance, in a related example, two installment locations are prepared in order to install a wireless device corresponding to the frequency range of 830 to 835 MHz and another wireless device corresponding to the frequency range of 900 to 905 MHz. By replacing these wireless devices with one wireless device corresponding to the frequency range of 830 to 905 MHz, it is possible to reduce the number of installment locations to one, while achieving an equivalent effect. It is desirable to configure the wireless device in this situation so as to be able to handle both a transmission (i.e., a transmission using a broadband signal) using both the band of 830 to 835 MHz and the band of 900 to 905 MHz at the same time and a transmission (i.e., a transmission using a narrowband signal) using only the band of 900 to 905 MHz. Further, to satisfy the wireless characteristics in both of the transmission modes, there is a demand for a distortion compensation method that is compatible with both the broadband signal and the narrowband signal.
In this situation, the more evenly the addresses in the look-up table are used, the higher a precision level of the distortion compensation becomes. There is, however, a possibility that there may be a difference in the distributions of the “parameter” values described above between the broadband signal and the narrowband signal. Thus, there is a possibility that there may be a difference in the distributions of the addresses. In other words, narrowband signals have a tendency of having a narrower distribution of addresses, and there is a possibility that the efficiency in the address allocation may be degraded. To cope with this situation, a method has already been proposed by which a “normalized gain” of addresses is changed in accordance with the state of the signal, so as to be able to evenly use the addresses in the look-up table.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-347944
However, when the “normalized gain” is set to too large a value, the reference signal per address becomes less. As a result, there is a possibility that the efficiency in the address allocation may, on the contrary, become degraded for the look-up table as a whole. In other words, there is a possibility that the precision level of the distortion compensation may, on the contrary, become degraded.